In mobile telephony, the telephone instrument is not connected permanently to the network but communicates with the exchange via a so called "air interface" and a base station which is connected to the exchange. The interface may be regarded as a switch capable of connecting a plurality of various telephone instruments to a base station and to the exchange.
In ordinary wired telephone systems the exchange can identify a certain telephone set by the particular telephone line used by the subscriber. Another subscriber who fraudulently wants to use the same line must connect the set to the line which is difficult if the fraudulent user is not skilled for such technical handling.
In a mobile telephone system there is no such connection problem. Instead a fraudulent users set having the same identification code as the genuine set can use the subscription of the latter without being detected if no measures are taken, and the genuine user is billed for the fraudulent users calls.
In order to prevent fraudulent use in a mobile telephone system a supervising method implying so called "rolling keys" has been proposed. The system uses a key "K" in the mobile which indicates the authentication of the mobile. There is a non-secret initial key "K.sub.i " and a new key "K.sub.n " can be calculated from K.sub.i and from a random number R provided to the mobile by the network. After that this key has been used, new keys K.sub.n+1 can be calculated from new generated random numbers. The rolling key system provides protection against fraud and may also be used to supervise that only one mobile uses the subscription. However it will normally not fully protect the subscriber from being charged for fraudulent calls since it is not the intention (and not economical) to roll the keys for each call.